Fire Tests
In March 2012, LTA of Singapore conducted a series of tests in the A86 Tunnel in Spain. These tests were per-formed with various standard drop nozzle configurations and water application rates. Three in particular were of interest for calibration purposes described in Table 1.
H20
Fuel (∆HT)
Flame
∆HW
∆Hg m”w, ex
m” q”
Legend
q” - heat source or net fl ux m” - mass loss rate of the fuel
∆Hg - magnitude of the heat required to vaporize the fuel
∆HT - the fuel souce
∆HW - heat of gasifi cation m”w, ex - critical water application rate
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Figure 1 - Dynamics of Fire and Extinguishment
Fire and Life Safety SES and Modeling
Computer Models
Computer models were developed in fire dynamics simu-lator (FDS) by the author for these three tests. The model quantities and reported test values are tabulated and summarized in Table 2. The grid size was chosen as a cube with lengths of 0.125 meters, a value that has been shown to give reasonable results in other simula-tions performed by the author.
Results
The results of the free-burning test show reasonable cor-relation between the model and the test for the heat re-lease rate and gauge heat flux as indicated in Figure 2.
The modeled peak heat release rate is slightly higher than the test. The growth rate is slightly faster than the test.
The extreme decay period is not considered significant because the major effects of the fire have passed. The modeled gauge heat flux is considerably more aggressive
than that measured. Gauge heat flux is measured with respect to some reference temperature in the gauge, of-ten determined by a cooling water feed. While there are peaks that are higher and lower than that measured, the overall magnitude reasonably tracks that of the fire and can be used for design purposes. Heat release rate can be used as an indicator of the fire power. The net heat flux was not measured. Net heat flux is the parameter used to determine if fuel vaporization can occur and with it result-ing target ignition. In both cases, the target ignited.
Gas temperatures were compared in Figure 3. For the un-suppressed fire, the model shows reasonable correlation with the test. For the suppressed fire, the model gas tem-peratures are lower than tested. However, both model and test showed temperatures too high for tenable conditions and low enough not to be a concern to the structural in-tegrity. This is reasonable correlation for design purposes.
Figure 2 – Comparison of model and test results for unsuppressed fire Heat Release Rate
Heat Flux (kW/m2)
Heat Flux (5m downstream of fi re) Free burning (Test 7)
Heat Release Rate (MW)
Time (sec) 200
150
100
0 500 1000 1500 2000
50
0
Time (sec) 200
250
150
100
0 500 1000 1500 2000
50
0
Target ignited
Model
Model Gauge
Model Net Test Gauge
Test
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Table 1 - Description of LTA Fire Tests
Table 2 - Tabulation and comparison of fuel quantities LTA Test No. Water Application Rate
(mm/min) Activation Time
after 60° C Peak Fire Heat Release Rate
(FHRR) (MW) Target Ignited? Max Target Heat Flux (kw/m2)
1 12 4 minutes 37.7 No 2
2 8 4 minutes 44.1 Unknown Unknown
7 0 none 150 Yes 225
Model Values Wood Plastic Total Test Values
Volume (m3)/% 7.6/82 1.7/18 9.3 80/20
Mass (kg)/% 3,410/67 1,711/33 5,121 5,000
Energy (GJ)/% 58.0/61 37.6/39 95.6 99.2
Total inc. Target (GJ) 117
Fire and Life Safety SES and Modeling
DECEMBER 2014 http://www.pbworld.com/news/publications.aspxFigure 3 – Comparison of model and test results for unsuppressed and 12 millimeter/minute suppressed fire
Gas temperature (10m downstream of fi re) Gas temperature (10m downstream of fi re)
Temperature (oC) Temperature (oC)
1500
1000
500
0 500 1000 1500 2000
0 0
100 200 300 400 500
Time (sec) Time (sec)
0
+L51 oMS1 +R51 +L51 oMS1 +R51
Model Temp 5m downstream of fi re Deluge operation
500 1000 1500 2000
Model Temp 5m downstream of fi re
500 0
10 20 30 40 50 60 70 80 90 100 120
110
0 1000 1500 2000
0 500 1000 1500 2000
Time (sec) Time (sec)
With deluge system (Test 1)
Heat fl ux 5m downstream of fi re Heat Release Rate
Deluge operation
Deluge operation
Target not ignited
Heat Release Rate (MW) Heat Flux (kW/m2) Model Heat Flux (kW/m2)
0 0.5 1.0 1.5 2.0
0 5 10 15 20 25 30 35 40 45 50
Model gauge
Model net
Test
Heat fl ux 5m downstream of fi re
Test gauge Model
Figure 4 – Comparison of model and test results for 12 millimeter/minute suppressed fire
Fire and Life Safety SES and Modeling
The fire heat release rate (FHRR) and heat flux test re-sults for the 12 millimeter/minute (mm/min) suppressed fire, as shown in Figure 4, did not compare well at all. The model heat release rate was calculated as considerably higher than that tested. This is acceptable for design pur-poses, as the calculation indicates a higher value than measured. The reverse would be problematic.
The model gauge heat flux was also calculated as higher than the measured target gauge flux. The net heat flux was much closer to the gauge flux. In the model, like the test, insufficient target fuel vaporization occurred, resulting in no target ignition.
It should be remembered that the purpose of this work is to develop a spray system that meets a particular ob-jective. Fire point theory shows that net heat flux is the key parameter for predicting water effectiveness and understanding this allows for better predicting of spray performance. In the case of comparison with the LTA tests, this modeling exercise showed reasonably good correlation with the unsuppressed test for heat flux, and FHRR profile, as well as gas temperatures. In the case
of the suppressed test, the model showed higher val-ues than the test, but still showed that spread to the target fuel pile was prevented.
Conclusion
Full-scale testing of fire suppression systems is expen-sive. Computer modeling provides a cost-effective means of demonstrating proposed system performance. The fuel vaporization process is well-defined in fire science and the computer models can be structured to utilize this approach.
Comparison with a test is beneficial to calibrate the model. For this reason, the LTA tests are a significant milestone in providing a benchmark to compare model results and their contribution to the knowledge of the industry is extremely important.
Kenneth Harris, PE is a Tunnel Mechanical and Fire Protection Specialist and Principal Professional Associate with 40 years of experience in design, construction and inspection of large civil and industrial projects.1
1Kenneth Harris has written a number of articles for Network including “Hydraulic Modeling of Fire Protection Pipelines for the Westside Rail Tunnel” Network #34, Spring 1996, pp 24, 25.
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